Advanced processing of audio signals has become increasingly important in many areas including e.g. telecommunication, content distribution etc. For example, in some applications, such as teleconferencing, complex processing of inputs from a plurality of microphones has been used to provide a configurable directional sensitivity for the microphone array comprising the microphones. Specifically, the processing of signals from a microphone array can generate an audio beam with a direction that can be changed simply by changing the characteristics of the combination of the individual microphone signals.
Typically, beam form systems are controlled such that the attenuation of interferers is maximized. For example, a beam forming system can be controlled to provide a maximum attenuation (preferably a null) in the direction of a signal received from a main interferer.
A beam form system which provides particularly advantageous performance in many embodiments, is the Filtered-Sum Beamformer (FSB) disclosed in WO 99/27522.
In contrast to many other beam forming systems, the FSB system seeks to maximize the sensitivity of the microphone array towards a desired signal rather than to maximize attenuation towards an interferer. An example, of the FSB system is illustrated in FIG. 1.
The FSB system seeks to identify characteristics of the acoustic impulse responses from a desired source to an array of microphones, including the direct field and the first reflections. The FSB creates an enhanced output signal, z, by adding the desired part of the microphone signals coherently by filtering the received signals in forward matching filters and adding the filtered outputs. Also, the output signal is filtered in backward adaptive filters having conjugate filter responses to the forward filters (in the frequency domain corresponding to time inversed impulse responses in the time domain). Error signals are generated as the difference between the input signals and the outputs of the backward adaptive filters, and the coefficients of the filters are adapted to minimize the error signals thereby resulting in the audio beam being steered towards the dominant signal. The generated error signals can be considered as noise reference signals which are particularly suitable for performing additional noise reduction on the enhanced output signal z.
A particularly important area for audio signal processing is in the field of hearing aids. In recent years, hearing aids have increasingly applied complex audio processing algorithms to provide an improved user experience and assistance to the user. For example, audio processing algorithms have been used to provide an improved signal to noise ratio between a desired sound source and an interfering sound source resulting in a clearer and more perceptible signal being provided to the user. In particular, hearing aids have been developed which include more than one microphone with the audio signals of the microphones being dynamically combined to provide directivity for the microphone arrangement. As another example, noise canceling system may be applied to reduce the interference caused by undesired sound sources and background noise.
The FSB system promises to be advantageous for applications such as hearing aids as it promises an efficient beam forming towards a desired signal (rather than being directed to attenuation of interfering signals). This has been found to be of particular advantage in hearing aid applications where it has been found to provide a signal to the user which facilitates and aids the perception of the desired signal. In addition, the FSB system provides a noise reference signal which is particularly suitable for noise reduction/compensation for the generated signal.
However, it has been found that the FSB system has some associated disadvantages when used in applications such as for a hearing aid. In particular, it has been found that for low distances between the microphones of the microphone array, the performance of the FSB system degrades. For example, for a typically hearing aid configuration of an end-fire array with two omni-directional microphones with a spacing of 15 mm, the FSB has been found to have suboptimal performance. Indeed, it has been found that in many scenarios, the FSB system has not been able to converge towards the desired signal.
Hence, an improved audio beam forming would be advantageous and in particular a beam forming allowing improved suitability for hearing aids for which distance between microphones is rather small.